1. Field of the Invention
This invention relates to the oxidation of hydrocarbons and is more particularly concerned with hydrocarbon oxidation employing an improved oxygen-injection method.
2. Description of the Prior Art
Glycol esters, and especially glycol carboxylic acid esters, are particularly useful organic chemical intermediates and extractive solvents. The glycol esters produced from ethylene are useful in the production of ethylene glycol an important commercial chemical. Catalytic processes for the preparation of the glycol esters have been disclosed, for example, in U.S. Pat. No. 3,689,535 and in Belgian Pat. No. 738,463. Ethylene glycol may be prepared by the hydrolysis of the ethylene glycol carboxylic acid esters, as disclosed in U.S. pat. No. 3,647,892, and vinyl acetate may be prepared by pyrolysis of the ethylene glycol ester, as disclosed in U.S. Pat. No. 3,689,535. Similarly, propylene glycol may be prepared by the hydrolysis of propylene glycol carboxylic acid esters, and other glycol esters yield the corresponding glycols upon suitable hydrolysis. (Belgian Pat. No. 862,054 (issued 1978) relates to an improved process for preparing glycol esters in which olefin, molecular oxygen, and a carboxylic acid are contacted in the presence of a catalyst system comprising a variable valent cation in association with bromine, chlorine, a bromine-containing compound or a chlorine-containing compound in a vertically-elongated reaction zone provided with a branch vertical circulation zone for continuously transferring liquid from the upper portion of the vertical reaction zone to the bottom of the reaction zone, to provide predetermined vertical superficial liquid velocities, which disclosure is herein incorporated by reference.
While the known processes for oxidizing hydrocarbons, such as those disclosed in the above-mentioned patents and patent applications, are effective for the indicated purposes, they are susceptible to improvement from the standpoint of optimum operation with continuous oxygen feeds to the reactor, generally by use of gas spargers. However, such devices which sparge gas into a liquid phase reaction medium containing the hydrocarbon reactants are quite vulnerable to loss of oxygen flow caused, for example, by a discontinuity in the feed pressure of the oxygen gas to the system. In such a case, the danger exists of back flow of part of the liquid reaction medium into the gas sparger. In start-up of the reactor following this discontinuity of oxygen feed, the spargers must be flushed to insure that the flammable hydrocarbons are removed from the spargers prior to passage of oxygen gas therethrough for safety's sake. This results in added equipment and process time expenses. In the event the flammable hydrocarbons are flushed using an inert gas, such as nitrogen, the large volumes of gas employed must be removed from the reactor's effluent gases and offer a potential environmental pollution source which must be carefully treated before discharging to the atmosphere. Use of a nonflammable flushing liquid which is inert to the components in the reactor can introduce impurities to the system, and, again, the volumes of flushing liquid can be expensive to recover from the reactor's product streams. The dilution of the reaction media resulting from use of such flushing liquids can also adversely affect reactor efficiency during startup, decreasing product yields, selectivity and quality. To avoid introduction of impurities in the system, the non-flammable liquid which would ordinarily be used to flush the spargers would be one which would be indigenous to the process. However, such liquids are themselves not entirely satisfactory. For example, in the case of the preparation of vicinal glycol esters by the above-described olefin-oxidation process, the only non-flammable liquid indigenous to the process is water, and it has been found that significant quantities of water, when passed through the gas spargers to flush them free of the hydrocarbon reactants, results in the deposit on the gas spargers and other process equipment of a resin-like material which is believed to result from the reaction of water with components of the liquid medium. Since this deposited resin-like material can readily plug the spargers and is organic in nature, the flushing of the gas spargers with water is unacceptable.